Datasheet

LM4780, LM4780TABD
SNAS193B –JULY 2003–REVISED APRIL 2013 www.ti.com
With large values of R
IN
oscillations may be observed on the outputs when the inputs are left floating. Decreasing
the value of R
IN
or not letting the inputs float will remove the oscillations. If the value of R
IN
is decreased then the
value of C
IN
will need to increase in order to maintain the same -3dB frequency response.
HIGH PERFORMANCE CONSIDERATIONS
Using low cost electrolytic capacitors in the signal path such as C
IN
and C
i
(see Figure 1 to Figure 6) will result in
very good performance. However, electrolytic capacitors are less linear than other premium capacitors. Higher
THD+N performance may be obtained by using high quality polypropylene capacitors in the signal path. A more
cost effective solution may be the use of smaller value premium capacitors in parallel with the larger electrolytic
capacitors. This will maintain signal quality in the upper audio band where any degradation is most noticeable
while also coupling in the signals in the lower audio band for good bass response.
Distortion is introduced as the audio signal approaches the lower -3dB point, determined as discussed in the
section above. By using larger values of capacitors such that the -3dB point is well outside of the audio band will
reduce this distortion and improve THD+N performance.
Increasing the value of the large supply bypass capacitors will improve burst power output. The larger the supply
bypass capacitors the higher the output pulse current without supply droop increasing the peak output power.
This will also increase the headroom of the amplifier and reduce THD.
SIGNAL-TO-NOISE RATIO
In the measurement of the signal-to-noise ratio, misinterpretations of the numbers actually measured are
common. One amplifier may sound much quieter than another, but due to improper testing techniques, they
appear equal in measurements. This is often the case when comparing integrated circuit designs to discrete
amplifier designs. Discrete transistor amps often “run out of gain” at high frequencies and therefore have small
bandwidths to noise as indicated below.
Integrated circuits have additional open loop gain allowing additional feedback loop gain in order to lower
harmonic distortion and improve frequency response. It is this additional bandwidth that can lead to erroneous
signal-to-noise measurements if not considered during the measurement process. In the typical example above,
the difference in bandwidth appears small on a log scale but the factor of 10in bandwidth, (200kHz to 2MHz) can
result in a 10dB theoretical difference in the signal-to-noise ratio (white noise is proportional to the square root of
the bandwidth in a system).
In comparing audio amplifiers it is necessary to measure the magnitude of noise in the audible bandwidth by
using a “weighting” filter
(1)
. A “weighting” filter alters the frequency response in order to compensate for the
average human ear's sensitivity to the frequency spectra. The weighting filters at the same time provide the
bandwidth limiting as discussed in the previous paragraph.
In addition to noise filtering, differing meter types give different noise readings. Meter responses include:
1. RMS reading,
2. average responding,
3. peak reading, and
4. quasi peak reading.
Although theoretical noise analysis is derived using true RMS based calculations, most actual measurements are
taken with ARM (Average Responding Meter) test equipment.
(1) CCIR/ARM: A Practical Noise Measurement Method; by Ray Dolby, David Robinson and Kenneth Gundry, AES Preprint No. 1353 (F-3).
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